Solid-shell centrifuges of the kind here in question, work, for example, as decanters in separation processes for clarification, dewatering, wet classification, solid-liquid-extraction and such like processes. In this connection, a scroll rotates with slight gap inside a bowl shell and in the same direction. The scroll operates by means of a speed slightly different from that of the bowl shell as the conveyer element for the removal of relatively dry residue, whilst the clarified liquid on the inner shell of the bowl flows off in the opposite direction.
In order to control the separation result of the centrifuge, the differential speed or scroll speed or the height of weir plate or the amount of inflow can be controlled.
For such a control, the scroll torque has hitherto been the most important command variable. A reason therefor lies in the relative simplicity of obtaining this information, depending on the driving system, as electric or hydrostatic variable. In addition to this, with incompressibles, that is solid matter which is not further compressible, the torque is a statement of the degree of loading of the machine with solid matter and the amount thereof in the drying stage. With compressible solids, the torque is a statement which is made up on the one hand from the filling rate, and on the other hand from the degree of concentration of the solids and the shearing resistance thereof.
As a rule, the scroll torque is employed as the command variable. The reason therefore lies on the one hand in the fact that the arrangement for acquiring the torque can usually also be used as the regulating unit of the control; on the other hand, that the scroll speed control is a control which ensures the flexibility of the centrifuge over a wide field of operations at a simultaneous optimization of the results pertaining to separation engineering.
In the marginal areas of the torque, be it at extremely low torque caused by very flowable sediment, or also at extremely high torque caused by a very high concentration, difficulties occur with control systems that have the torque as command variable, which restricts the field of operation drastically or may even lead to a total breakdown of the control. With applications in which the solids phase still contains very great flow qualities, the determined torque is so low that the value thereof lies far below the mean level of the disturbance variables, which are generated by friction from individual larger particles which may be present. With applications where the degree of concentration of the solids becomes high, the control that is dependent on the torque fails as a system. Indeed, such a control is a control system by which the torque is the output variable as well as also being the input variable of the control, thus a control system with feedback, which is clearly subject to reactive coupling. The inherent frequency of such a control system which starts to oscillate, depends on the time constant of the system. By a scroll speed control dependent on the torque, the concentration time is decisive; by a feed control dependent on the torque, it is the sedimentation time and the concentration time. As long as the sediment in the storage area of the machine possesses good flow qualities, this mass is capable of "swallowing" every disturbance variable from the feed, which thus means that everywhere in this mass, the concentration, the solidification and the shearing resistance increases monotonic in centrifugal direction. If, however, the flow qualities of the sediment sink, which is the rule upon increasing solidification, this mass hence begins to develop a "memory" for feed-end disturbance variables, and the tendency towards an oscillating of the control system increases. A reduction of the control factor or intensifier factor of the control system, or the attenuation of the command variable, restrict the field of operation of the control to such an extent that the latter is no longer capable of handling a temporary larger accumulation of solids, and thus fails one of the most important purposes thereof.